bertran,
Like WJD pointed out, the username is username (no @domainname.com part) and the password is the password you use for your Site Manager/main mail account/FTP/SSH.
However, if you still cannot access it, the easy way for you to "reset" the password is to FTP or SSH in to your account, change to the www/username/plugins/PhpMyAdmin/ directory and delete (better to rename for now) the file .htaccess (example: backup.htaccess). This way, the password protection for the directory will be removed. Sure it is really unsecure, and you should definitely not leave it like that, but you can always re-generate a user/pass combo yourself (and at least for the time being you will be able to access it).
See this post for details: http://forums.westhost.com/phpBB2/viewtopic.php?t=603